Continuous measurement of the fineness of a pulverulent material

ABSTRACT

In order to measure the degree of fineness of a pulverulent material, the pulverulent material is placed in suspension in a gaseous fluid and a converging laser beam from an optical source is passed through the fluid. The luminous flux of the diffracted light is measured by two or more photoelectric cells in the focal plane of the optical system, the cells being at different distances from the optical axis. The fineness of the material is calculated on the basis of the result of these measurements.

United States Patent 1 [1 3,834,818

Meric Sept. 10, 1974 [54] CONTINUOUS MEASUREMENT OF THE 2,816,47912/1957 Sloan 356/104 FINENESS OF A PULVERULENT 3,328,587 6/1967 Brownet al. 356/104 MATERIAL 3,462,608 8/1969 Weston et a1... 250/2183,467,471 9/1969 Greenfield 356/36 [75] Inventor: Jean Paul Meric,Paris, Fran 3,705,771 12/1972 Friedman et al 356/104 [73] Assignee:Centre DEtudes Et De Recherches De Llndustrie Des Liants PrimaryExaminer-Vincent P. McGraw HydrauIiques, Paris, France Attorney, Agent,or Firm-Eric H. Waters [22] Filed: Mar. 26, 1973 [21] Appl. No.: 345,199[57] ABSTRACT In order to measure the degree of fineness of a pulver-Forelgn Application Dam ulent material, the pulverulent material isplaced in Mar. 29, 1972 France 72.11018 suspension in a gaseous fluidand a converging laser beam from an optical source is passed through the[52] US. Cl 356/102, 356/36, 356/ 104 fluid. The luminous flux of thediffracted light is mea- [51] Int. CL... G01n 15/02, G01n 21/00, GOln1/00 sured by two or more photoelectric cells in the focal [58] Field ofSearch 356/36, 102, 104, 207; plane of the optical system, the cellsbeing at different 250/218 distances from the optical axis. The finenessof the material is calculated on the basis of the result of [56]References Cited these measurements.

UNITED STATES PATENTS 2,732,753 1/1956 OKonski 250/218 13 6 DrawmgFigures CONTINUOUS MEASUREMENT OF THE FINENESS OF A PULVERULENT MATERIALIt is known to determine the granulometry of a pulverulent material byforming in the focal plane of an optical system the diffraction spot ofa sample of the material illuminated by a laser beam, and measuring theluminous flux of n coronas concentric with this diffraction spot.

This method allows very exact determination of the granulometry of thepulverulent material, but it is relatively complicated in that itrequires the solving of n linear equations with n unknowns and may notbe operated continuously unless a computer is used.

The object of the present invention is to provide a method whichfacilitates determination of the fineness of a pulverulent material, butwhich is simpler and more easily operated on a continuous basis.

In the method according to the present invention the pulverulentmaterial is placed in suspension in a gaseous fluid, a converging laserbeam issuing from an optical system is passed through this fluid, thediffracted light is measured at at least two points situated in thefocal plane of the optical system, outside the source of light of thesystem, and at different distances from this source of light, and thefineness is calculated on the basis of the result of these measurements.

When the fluid does not contain powder the diffracted light received atthe two points is very weak. On the other hand, if it contains powder,the luminous fluxes measured are due to diffracted light. If the powderis fine (compared with the wavelength) the flux is greater at the pointmore remote from the optical axis, whereas if the powder is coarse theflux is greater at the point nearer to the axis. Tests show that theratio of the two measurements depends neither on the intensity of thelaser nor on the concentration of the powder in the gas. This ratiodepends solely on the fineness of the powder and is thus representativeof the fineness.

To express it more precisely, the measurement S, obtained at the pointmore remote from the axis is a function of the proportion of materialwhose particles have a diameter less than a certain value d,. On theother hand, whatever the diameter of the particles, a certain proportionof the light is diffracted at the point nearer to the axis to give ameasurement 5,; but by far the greatest part of this light is due toparticles with diameter exceeding a certain value d The ratio S, (S, Sthus represents the proportion of particles in the material which has adiameter smaller than a value d lying between d, and d it is thuspossible to determine a point on the granulometric division curve. Inpractice, as these measurements are relative, one may be content withtaking the value S,/S

The diffracted light may be measured at three points to give threemeasurements 8,, S S The two ratios S, (S S and S,,(S, S then make itpossible to determine two points on the granulometric division curve.

More generally it is possible to measure the diffracted light at two ormore points so as to obtain a series of measurements 8,, S S,, and toapply a formula such as fineness =(a, S +112 S2 a S /b 51+ [72 S2 S")the various coefficients being determined by calculation or experiment.This calculation may be carried out with the help of an analog ordigital circuit and the result may be shown directly.

The present invention also provides apparatus for carrying out themethod described above.

This apparatus comprises means for placing the pulverulent material insuspension in a gaseous fluid, means for generating a laser beam, anoptical system concentrating the laser beam and disposed in such a waythat this converging beam passes through the liquid, and at least twophotoelectric cells disposed in the focal plane of the optical system,outside the source of light of this system and at different distancesfrom this source of light.

The distances of the cells from the optical axis may, in principle, bewhatever is desired. However, they depend on the point on thegranulometric curve by which it is desired to characterize the fineness.The half-angle s at the apex of the cone of diffracted light clue toparticles with a diameter d expressed in pm is given by s 0.8/d radians.

In order to express the fineness as the proportion of particles of thematerial having a diameter less than d, it is sufficient to place onecell inside a cone with s for the half-angle at the apex and the secondcell outside this cone. In the case of cement it is preferable to choosefor d a value lower than 10 pm, for example 8 am, in which case thevalue of s is 0.1 radian.

The invention will be described further by way of example only, withreference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic representation of apparatus for determining thefineness of a pulverulent material;

FIG. 2 is a diagrammatic vertical section through a device used to placethe pulverulent material in suspension;

FIG. 3 shows vertical section an arrangement for supplying pulverulentmaterial to the device of FIG. 2;

FIG. 4 is a horizontal sectional view of the arrangement of FIG. 3;

FIG. 5 is a sectional view along line VV of FIG. 3; and

FIG. 6 is a sectional view along line VI-Vl of FIG. 5, the groove beingshown developed.

With reference to FIG. I, a gaseous jet 1, contains in suspension apulverulent material whose fineness is to be determined. The apparatusused for this purpose comprises a laser 2 in front of which is placed aspatial filter 3. This filter purifies the beam by suppressing thenon-parallel rays, broadens it, directs it through the jet 1, andfocusses it at a point where a diffuser 4, constituted for example by asmall point or a black cone, is disposed. In the plane P of thisdiffuser, in other words in the focal plane of the spatial filter 3, aredisposed a series of photoelectric cells, which in this case number two,5a and 5b, but the number may be greater. Of these two cells, one cell5a is placed in the inside of the diffraction cone having an apex 0 inthe jet 1 and a halfangle s at the apex s equal to 0.1 radian, and theother cell 512 is outside this cone.

Each of these cells supplies a signal ,S, or S and the fineness is givenby the ratio of S,/S

In order to place the pulverulent material in suspension, the deviceshown in FIG. 2 may be used. This device comprises a duct or chamber 6into which extend a compressed air inlet conduit 7 and a conduit 8 whoselower end is immersed in a fluidized bed 9 of the pulverulent material,for example, cement.

A convergent-divergent nozzle 10 is disposed at the output of the ductor chamber 6 and is surrounded by a tube 11 forming a passage forsecondary air discharging at the end of the nozzle 10. A blower 12 isplaced in front of this convergent-divergent nozzle 10. The laser beampasses between the tube 11 and the blower 12, as is showndiagrammatically by the arrow 24.

In operation, the pulverulent material is drawn into the duct or chamber6 through the conduit 8. Here the material passes across a series ofstationary supersonic shockwaves which are produced by theconvergentdivergent nozzle 10, the effect of these being to disperse anddeflocculate the material. At the outlet of the nozzle, the material isin the form of a homogeneous suspension delimited by the flux of cleansecondary air. This flux is used to prevent the powder laden jetbursting and makes it possible to localize this jet very precisely. I

The blower 12 evacuates the suspension which has just passed across thelaser beam issuing from the spatial filter 3.

Instead of taking samples of the pulverulent material directly from areceptacle containing a fluidized bed of material, it is also possibleto proceed as is shown in FIGS. 3 to 6.

A sampling element 13, consisting of, for example, a motorized screw ora pneumatic guide channel, transfers into an inclined conduit 14 a smallfraction of the pulverulent material. This conduit 14 discharges into afunnel 15 provided with a slot 16 through which a disc 17 projects. Thisdisc is fixed on a vertical axle 18, which is supported by bearings 19and is connected to the output shaft of a motor 20 (if necessary via aspeed reduction means), so that the disc 17 rotates in the directionshown by the arrow f.

The disc 17 has an annular groove 21 which has a triangular section anda rounded bottom, and the funnel 15 contains scrapers 22 (or a singlescraper) arranged in such a way that they level the material in thegroove, prevent the material contained in the funnel from escapingthrough the slot 16, and discharge the material over the edge of thedisc.

A hollow needle 23, whose extremity 23a is shaped as a bevel, isintroduced into the groove 21 so that its bevel is turned forwards,i.e., it faces in the direction of rotation of the disc 17. This needleis connected to the conduit 8 (FIG. 2).

In operation, the pulverulent material introduced into the funnel 15drops onto the disc 17 and fills the groove 21. The material is smoothedby the scrapers 22, so that the groove is full when it emerges from thefunnel. The excess material drops from the disc into the bottom of thefunnel 15 where it is evacuated.

The needle 23 ploughs along the groove 21 and sucks up the materialcontained in the groove, under the action of the reduced pressurecreated in the conduit 8, through which it then passes. The bevelledshape of the extremity 23a of the needle, and the fact that the bevel isdirected forwards, ensure regular feeding of the needle and prevent thematerial being sucked up in jerks.

emptied thanks to its triof the material from the result of thesemeasurements.

2. A method as claimed in claim 1, in which one of the said points liesinside a diffraction cone whose halfangle at the apex is 0.8/d radian, dbeing the material particle diameter selected to characterize thefineness of the material, another of the said points lying outside thesaid cone.

3. A method as claimed in claim 2, in which the said half-angle is 0.1radian.

4. Apparatus for determining the fineness of a pulverulent material,comprising means for placing the pulverulent material in suspension in agaseous fluid, a laser for generating a laser beam, an optical systemarranged to cause the laser beam to converge and for the convergent beamto pass through the suspension of pulverulent material in the gaseousfluid, and at least two photoelectric cells disposed in the focal planeof the optical system, outside the focus of the beam and at differentdistances from this focus.

5. Apparatus as claimed in claim 4, in which one of the cells isdisposed inside a diffraction cone whose half-angle at the apex is 0.8/dradian, d being the material particle diameter selected to characterizethe fineness of the material, another of the cells being disposedoutside the said cone.

6. Apparatus as claimed in claim 5, in which the said cone has ahalf-angle at the apex of 0.1 radian.

7. Apparatus as claimed in claim 4, in which the means for placing thepulverulent material in suspension comprises a compressed air ducthaving an inlet for the material, and a convergent-divergent nozzle atthe outlet of the duct.

8. Apparatus as claimed in claim 7, in which the nozzle is surrounded bya secondary air inlet conduit discharging at the extremity of thenozzle.

9. Apparatus as claimed in claim 7, in which the means for placing thepulverulent material in suspension further comprises a disc having anannular groove, means for rotating the disc, means for supplying thepulverulent material onto the disc so that it fills the groove, and ahollow needle, one end of which extends into the groove whereas itsother end is connected to the material inlet of the compressed air duct.

10. Apparatus as claimed in claim 9, in which the groove has atriangular cross-section with a rounded bottom.

11. Apparatus as claimed in claim 9, in which the end of the needle inthe groove is provided with a bevel facing in the direction of rotationof the disc.

12. Apparatus as claimed in claim 9, in which the means for supplyingthe pulverulent material comprises a funnel whose wall has a slotthrough which the disc projects into the funnel.

13. Apparatus as claimed in claim 9, further comprising at least onescraper arranged to level the material in the-groove.

1. A method of determining the fineness of a pulverulent material,comprising: placing the pulverulent material in suspension in a gaseousfluid; passing through the fluid a converging laser beam issuing from anoptical system, the beam being diffracted by the material; measuring theluminous flux of the diffracted light at at least two points situated inthe focal plane of the optical system, outside the focus of the beam,and at difFerent distances from this focus; and calculating the finenessof the material from the result of these measurements.
 2. A method asclaimed in claim 1, in which one of the said points lies inside adiffraction cone whose half-angle at the apex is 0.8/d radian, d beingthe material particle diameter selected to characterize the fineness ofthe material, another of the said points lying outside the said cone. 3.A method as claimed in claim 2, in which the said half-angle is 0.1radian.
 4. Apparatus for determining the fineness of a pulverulentmaterial, comprising means for placing the pulverulent material insuspension in a gaseous fluid, a laser for generating a laser beam, anoptical system arranged to cause the laser beam to converge and for theconvergent beam to pass through the suspension of pulverulent materialin the gaseous fluid, and at least two photoelectric cells disposed inthe focal plane of the optical system, outside the focus of the beam andat different distances from this focus.
 5. Apparatus as claimed in claim4, in which one of the cells is disposed inside a diffraction cone whosehalf-angle at the apex is 0.8/d radian, d being the material particlediameter selected to characterize the fineness of the material, anotherof the cells being disposed outside the said cone.
 6. Apparatus asclaimed in claim 5, in which the said cone has a half-angle at the apexof 0.1 radian.
 7. Apparatus as claimed in claim 4, in which the meansfor placing the pulverulent material in suspension comprises acompressed air duct having an inlet for the material, and aconvergent-divergent nozzle at the outlet of the duct.
 8. Apparatus asclaimed in claim 7, in which the nozzle is surrounded by a secondary airinlet conduit discharging at the extremity of the nozzle.
 9. Apparatusas claimed in claim 7, in which the means for placing the pulverulentmaterial in suspension further comprises a disc having an annulargroove, means for rotating the disc, means for supplying the pulverulentmaterial onto the disc so that it fills the groove, and a hollow needle,one end of which extends into the groove whereas its other end isconnected to the material inlet of the compressed air duct. 10.Apparatus as claimed in claim 9, in which the groove has a triangularcross-section with a rounded bottom.
 11. Apparatus as claimed in claim9, in which the end of the needle in the groove is provided with a bevelfacing in the direction of rotation of the disc.
 12. Apparatus asclaimed in claim 9, in which the means for supplying the pulverulentmaterial comprises a funnel whose wall has a slot through which the discprojects into the funnel.
 13. Apparatus as claimed in claim 9, furthercomprising at least one scraper arranged to level the material in thegroove.